In wireless local area networks governed by IEEE 802.11 standards, some frames may be retransmitted as part of channel access mechanism for contention-based medium access control to provide more reliable data transportation over a wireless link. Examples of retransmitted frames include individually addressed data frames and management frames as well as request-to-send (RTS) frames. Retransmissions may also occur for groupcast audio/video streams (e.g., in 8002.11aa and 802.11ad).
A retransmission may be triggered by one or more events. Examples of the events include a timeout that can occur if an acknowledgment (ACK) for a transmitted frame is not received (ACKTimeout), a timeout that can occur if a clear-to-send (CTS) frame in response to an RTS frame is not received (CTSTimeout), or receipt of a frame other than an expected ACK frame or CTS frame. Retransmissions may be performed using a backoff procedure with an exponentially increasing contention window. Retransmissions may be managed by using multiple retry counts and by limiting number of retries as specified in respective standards.
There can be many possible causes for retransmissions. For example, retransmissions can be caused by packet loss or packet error. Packet loss can occur when a receiver cannot decode a received packet at all. A packet error can occur when the receiver can decode part of a frame (e.g., a Physical Layer (PHY) preamble and Physical Layer Convergence Protocol (PLCP) header/signal (SIG) fields) but cannot correctly decode the entire frame (e.g., Frame Check Sequence (FCS) checking failures).
Packet loss or packet error can occur due to many reasons. Examples of the reasons include packet collisions. For example, packet collisions can be caused by hidden nodes and overlapping basic service sets (OBSSs). For example, in active scanning, a client station (STA) commonly sends a wildcard/broadcast probe request. In dense deployments, usually multiple access points (APs) will respond to the STA's wildcard request. If two of the APs are hidden nodes, then the probe response frames sent by the two APs may collide with each other at the STA causing the two APs to retransmit the probe response frames.
Inappropriate modulation and coding scheme (MCS) selection, particularly an aggressive MCS selection, can also cause packet loss or packet error. Aggressive selection of MCS can lead to potentially higher packet error rate (PER), more retransmissions, and lower goodput (i.e., (1−PER)*(Data Rate)). Other causes of packet loss and packet error include frame type specific issues. For example, an AP may continue to retransmit deassociation frames or deauthentication frames to a STA due to an absence of an ACK from the STA since the STA has already left the BSS of the AP.
Another cause for packet loss and packet error may be a weak uplink signal due to the fact that the STA is at a cell boundary and is moving away from the AP. In such a scenario, the AP may not receive an ACK transmitted by the STA. For example, the AP may not receive an ACK for probe response frames transmitted by the AP, which may cause the AP to retransmit the probe response frames. Also, if the STA is moving away from the AP but continues to associate (stick) with the AP, many retransmissions of uplink RTS frames can occur.